Introduction

Climate change is an increasing global concern. It is the adverse effect of the atmospheric accumulation of greenhouse effect gases as a result of human activity (1). Climate change is affecting every region of the globe, but in Asia and Africa, vulnerability is higher (2). Living organisms and biological processes associated with the transmission of infectious diseases are especially influenced by climatic variables such as humidity, precipitation, and temperature. The effects of climate change increase the density of vectors and transmission potential of many infectious diseases (3, 4).

Vector-borne diseases are caused by viruses, bacteria, and parasites that are spread by ticks, mosquitoes, tsetse flies, mites, snails, sand flies, triatomine bugs, black flies, and lice. Around 17% of all infectious diseases are vector-borne diseases. Every year there are more than 700,000 deaths globally from diseases such as yellow fever, Japanese encephalitis (JE), dengue fever, schistosomiasis, leishmaniasis, trypanosomiasis, chagas disease, onchocerciasis, and malaria (5, 6). Throughout the world outbreaks of vector-borne diseases have afflicted populations in many countries since 2014. In over 128 countries, more than 3.9 billion people are at risk. Mosquito-borne diseases cause an estimated 400,000 deaths world-wide every year. Leishmaniasis, chagas disease, and schistosomiasis effect hundreds of millions of people (7).

Pakistan is in the northwest of South Asia. It is situated between the latitudes 24-37º north and between the longitudes 62-75º east. It consists of four provinces (Balochistan, Khyber Pakhtunkhwa (KP), Punjab, and Sindh), the independent state of Azad Jammu and Kashmir (AJK), and 132 districts (8). There are four seasons. The onset and duration of seasons varies somewhat according to location. The eastern areas of the southern half mainly receive precipitation from June to September, while the northern and western areas of the southern half of the country get rains from December to March. The northern area includes some of world’s highest mountain peaks (K-2) and the largest glaciers (Siachen and Biafo). The temperature in winter in this region drops to as low as -50ºC and stays around 15ºC in the warmest months of May to September (9). The mean temperature during summer (March-June) varies from 23-49ºC (upper plain) and from 42-44ºC (lower plain), while the mean winter temperature (December-February) in the upper plain 20-23ºC and 14-20ºC in the lower plain (10).

The transmission of VBDs is determined by various factors, including social, environmental, and complex demographic factors, as well as factors relating to unplanned urbanization, trade, and both local and global travel. Changes in climate, such as changes in temperature and rainfall, can affect pathogen transmission and give rise to vector-borne diseases in new areas or regions and make the transmission season longer or more severe (11).

Methods

We searched all Web of Science databases for peer reviewed articles published between 1960 to December 2019. Search terms were related to vector-borne disease (dengue, malaria, chikungunya, CCHF, leishmaniasis, West Nile virus, Japanese encephalitis and yellow fever) present or historical occurrence in province (Balochistan, Punjab, Sindh, KP, Azad Jammu and Kahmir and Gilgit Baltistan) and climate change assessment. Only peer reviewed article published in English were included and year wise geographical distribution of vector borne diseases outbreaks in Pakistan were summarized. We did not find any published article related to occurrence of yellow fever therefore excluded it from the study.

Major vector-borne diseases in Pakistan

Mosquitoes are the best-known disease vector. The vectors usually swallow the pathogen while sucking blood from an infected host. During their later blood meal, the infected insects transmit the pathogen to a new host through bite. Some vectors of different diseases that are significant to public health are mosquitoes, ticks, fleas, flies, and sand flies (7).

The main VBDs in Pakistan are malaria, dengue, chikungunya, leishmaniasis, and West Nile fever.

Table 1

Vector Borne Diseases-their Pathogens and Vectors [12]

Disease Pathogen Vector Transmission
Malaria Plasmodium falciparum, vivax, ovale, malariae Anopheles spp. Mosquitoes Anthroponotic
Dengue DEN-1,2,3,4 flaviviruses Aedes aegypti Anthroponotic
Chikungunya Chikungunya fever virus, Alphavirus, Togaviridae Aedes aegypti and Ae. albopictus Anthroponotic
Leishmaniasis Leishmania spp. Lutzomyia & Phlebotomus spp. Sandflies Zoonotic Reservoir: Domestic animals - cow, buffalo, goat, dog
CCHF Arbo-virus genus Nairovirus (family Bunyaviridae) Hyalomma ticks Zoonotic

Climate change and its effects

Evidence of climate change in Pakistan

Pakistan faces a major climate change challenge. The Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC) for the South Asia region shows that warming is likely to be above the global mean and climate change will impact the glaciers’ melting rate and precipitation patterns, particularly affecting the timing and strength of monsoon rainfall. In the last 50 years, the annual mean temperature has increased by roughly 0.5°C and annual precipitation has shown a slight increase in the last 50 years. The annual mean temperature in Pakistan is expected to rise by 3°C to 5°C by the end of this century. Average annual rainfall is not expected to have a significant long-term trend. Under future climate change scenarios, Pakistan is expected to experience increased variability of river flows due to increased variability of precipitation and the melting of glaciers (10).

Effects of temperature on vectors and pathogens

The survival rates of vectors may increase or decrease depending on the species. For many diseases, transmission temperatures lie in the range of 14-18°C at the lower end 35-40°C at the upper end. At higher temperatures (30-32°C) there is a reduction in the extrinsic incubation period (EIP) of the pathogen; adult female mosquitoes digest blood faster and feed more and shorten the development period of the vectors leading to larger production of vector population, thus increasing transmission intensity. Temperatures above 34°C have a negative effect on the survival of vectors and pathogens (13, 14, 15). Moreover, temperature changes also bring changes in the transmission season and geographical distribution of disease vectors and VBDs, for example by rendering previously endemic areas unsuitable and previously non-endemic areas suitable for their existence and reproduction (16).

Effects of precipitation on vectors and pathogens

For vector-borne diseases, higher precipitation has been associated with dengue fever, malaria, scrub typhus, and Japanese encephalitis (17, 18). The survival may increase as increased rain may increase breeding places by increasing water storage and growth environments for vectors and hosts, such as mosquitoes, rodents, mites, and other insects (19, 20, 21). An important study indicated that mosquito density and disease transmission are susceptible to precipitation frequency. Another study showed that precipitation of ˃350 mm/day can eliminate vectors and is a positive effect (22). There are few direct effects, but humidity affects malarial parasite development (18).

Effects on transmission of VBDs

Dengue, chikungunya, CCHF and leishmaniasis are emerging in countries where they were previously unknown. Human activities in response to climate change strongly influence transmission. Local to international travel, trade, and climatic suitability for the vectors are the main factors responsible for the movement of vector-borne diseases (23, 24, 25). Furthermore, these factors may also introduce new vectors, pathogens, or hosts, causing some diseases to spread in areas where they are currently absent; for example, in Pakistan dengue has been endemic for decades in Karachi, but more recently dengue epidemics have emerged in Punjab in 2011. In 2017, a dengue outbreak occurred in Peshawar, Khyber Pakhtunkhwa. Dengue cases have been recorded from different districts of Sindh, Balochistan, KP, Punjab, and Islamabad. Climate change may affect the survival and reproduction rates of vectors, resulting in changes in the transmission patterns of pathogens and the geographical distribution of diseases.

Malaria

In Pakistan, malaria is a major cause of illness and death. Every year there are one million estimated and 300,000 confirmed reported cases. Pakistan has been grouped with Afghanistan, Somalia, Sudan, and Yemen in terms of the high burden of malaria in this region. A. stephensi and A. culicifacies are known primary vectors and P. Vivax and P. falciparum are the only reported parasite species in Pakistan. Transmission of malaria is seasonal. High transmission occurs in districts located in bordering regions with Balochistan, Sindh, the Islamic Republic of Iran, and Afghanistan. Balochistan and Khyber Pakhtunkhwa are the most malaria-endemic provinces. Thatta, Mirpur Khas, Khairpur, and Tharparkar are the major endemic districts of Sindh province. Punjab has been the lowest endemic province with API> 1/1000 (26).

Table 2

Year wise geographical distribution of Malaria cases in Pakistan

Year Cases Reported Area Reference
2004 40697 Badin, Hyderabad, Khairpur, Nawabshah, Nosheroferoz, Mirpur Khas, Mithi, Sanghar, Sukkur, Ghotki, Jacobabad, Shikarpur, Larkana, Dadu, Thatta, Karachi (27)
2005 27845 Badin, Hyderabad, Khairpur, Nawabshah, Nosheroferoz, Mirpur Khas, Mithi, Sanghar, Sukkur, Ghotki, Jacobabad, Shikarpur, Larkana, Dadu, Thatta, Karachi (27)
2004-06 652 Barkhan, Balochistan (28)
2004-06 443 Kohlu, Balochistan (28)
2009 4695 Punjab (29)
32403 Sindh (29)
25636 KP (29)
22056 FATA (29)
45435 Balochistan (29)
264 AJK (29)
2010 29046 Punjab (29)
69340 Sindh (29)
54278 KP (29)
32518 FATA (29)
66241 Balochistan (29)
316 AJK (29)
2011 19699 Punjab (29)
93306 Sindh (29)
75384 KP (29)
46149 FATA (29)
84579 Balochistan (29)
475 AJK (29)
2012 17522 Punjab (29)
114651 Sindh (29)
63494 KP (29)
30948 FATA (29)
63733 Balochistan (29)
433 AJK (29)
2013 9295 Punjab (29)
70269 Sindh (29)
98137 KP (29)
34116 FATA (29)
69678 Balochistan (29)
260 AJK (29)
2014 4993 Punjab (29)
47640 Sindh (29)
118512 KP (29)
35978 FATA (29)
67836 Balochistan (29)
190 AJK (29)
2015 3230 Punjab (29)
44728 Sindh (29)
65369 KP (29)
40494 FATA (29)
48021 Balochistan (29)
171 AJK (29)
2016 2868 Punjab (29)
63109 Sindh (29)
94100 KP (29)
88850 FATA (29)
74858 Balochistan (29)
239 AJK (29)
2017 81216 Thattha, Mirpur Khas, Khairpur, Sujawal, Umerkot, Sindh (30)
106915 DI Khan, Charsadda, Banu, Tank, KP (30)
80924 FATA (30)
72867 Jaffarabad, Zhob, Musakhel, Sherani, Balochistan (30)
2018 112224 KP (31)
111920 Sindh (31)
60896 Balochistan (31)
65853 Tribal Districts (31)
658 Punjab (31)

Dengue

Dengue is another mosquito-borne disease, causing mortality and illness in different regions of the world. It is estimated that dengue virus transmission is endemic in about 120 countries and 21,000 deaths are reported every year (32, 33). In Pakistan, dengue is endemic and transmission season has been observed in the post-monsoon period (34). Dengue fever was reported for the first time in 1985 (35). Pakistan experienced the worst dengue outbreaks during 2011, 2017 and 2019 (36, 37). In 2014, a dengue outbreak was reported from the Malakand district, KP [38]. Dengue cases were reported from provinces of Sindh, KP, and Punjab in 2016 (39). The Aedes mosquito vector of dengue is highly sensitive to climate conditions and studies suggest that climate change is likely to continue to increase exposure to dengue (16).

Figure 1 

Map showing the epidemiology of dengue in Pakistan currently and diachronically.

Source: https://wdrfree.com/stock-vector/pakistan-map-with-districts with modification.

Table 3

Year wise geographical distribution of dengue cases in Pakistan

Year Reported cases Area Mortality Reference
1994 145 Karachi 1 (40)
1995 75 Hub, Baluchistan 57 (41)
2003 1000 Haripur, KP 7 (42)
2004 2500 Khushab, Nowshera 11 (42)
2005 500 Karachi 13 (42)
2006 5400 Karachi, Sukkar, Nawabshah, Rawalpindi, Islamabad 55 (43)
2007 2700 Karachi, Hyderabad, Mirpur Khas, Lahore, Haripur, Rawalpindi, Islamabad 24 (44)
2008 1800 Lahore 30 (44, 45)
2009 570 Lahore 13 (44, 35)
2010 11024 Lahore, Sheikhupura, Gujranwala 40 (46)
2011 20685 Lahore 350 (41)
2012 712 Swat 48 (47)
2013 2180 Swat 72 (47)
2014 2999 Throughout the country 15 (47)
2015 9899 Punjab, Sindh, KP 6 (47)
2017 111979 Peshawar, KP 65 (41, 48)
142 Abbottabad, KP 4 (48)
143 Buner, KP - (48)
64 Haripur, KP - (48)
240 Malakand, KP - (48)
72 Mansehra, KP - (48)
369 Mardan, KP 1 (48)
199 Nowshehra - (48)
133 Swabi, KP - (48)
2884 Sindh - (49)
579 Punjab - (49)
86 Balochistan - (49)
06 AJK - (49)
120 ICT - (49)
406 Tribal districts-KP - (49)
2018 332 KP - (49)
175 Tribal districts-KP - (49)
2088 Sindh - (49)
69 Balochistan - (49)
539 Punjab - (49)
2019 13,294 Islamabad 22 (37)
10,119 Punjab 23 (37)
16,685 Sindh 46 (37)
7,082 Khyber Pakhtunkhwa 0 (37)
794 KP Tribal districts 0 (37)
3,474 Balochistan 3 (37)
1690 AJK 1 (37)

Chikungunya

In Pakistan, the chikungunya virus was detected in rodents in 1983 [50]. Aedes aegypti and Aedes albopictus are the main vectors of chikungunya. In 2011, during a dengue outbreak, a few chikungunya patients were also reported. Around 30,000 cases of chikungunya were identified in Karachi in 2011. More than 4,000 samples were found positive after lab diagnostic tests at the National Institute of Health (NIH) and Armed Forces Institute of Pathology, Pakistan (51). According to WHO, 1018 suspected cases of chikungunya were registered in 2017 from different districts of Sindh. A total of 157 suspected cases were confirmed for chikungunya infection by the NIH (52). A total 8521 suspected cases of chikungunya were reported from several areas of Pakistan from 2016 to 2018. The most affected provinces were Balochistan and Sindh (53).

Table 4

Year wise geographical distribution of Chikungunya cases in Pakistan

Year Reported cases Areas Reference
2016-18 4714 Karachi -54
2866 Balochistan -54
785 Tharparkar, Sindh -54
136 Khyber Pakhtunkhwa -54
18 Punjab -54
1 Azad Kashmir -54
2018 776 Sindh, Punjab, Khyber Pakhtunkhwa, Balochistan -53

Crimean-Congo haemorrhagic fever

In Pakistan, Crimean-Congo hemorrhagic fever (CCHF) was reported for the first time in 1976 and 14 cases were reported from 1976 to 2010 (55). About sixty-two assumed cases were identified during the year 2012, with forty-one confirmed and eighteen deaths; whereby thirteen deaths were laboratory-identified cases and five deaths were recorded as assumed CCHF. About twenty-three clear-cut cases were reported from Balochistan, seven from Sindh; six from Khyber Pakhtunkhwa and five from the Punjab province of Pakistan (56). In 2017, lab-confirmed CCHF cases were reported from the districts of Attock, Chakwal, Faisalabad, Gujrat, Kasur, Mianwali, Multan, Muzaffarghar, and Rawalpindi of Punjab. In the same year CCHF cases were repo reported from the provinces of Balochistan, Khyber Pakhtunkhwa, Karachi, and ICT (57). During 2018, confirmed CCHF cases reported from throughout the country (53).

Figure 2  

Map showing the epidemiology of CCHF in Pakistan currently and diachronically.

Source: https://wdrfree.com/stock-vector/pakistan-map-with-districts with modification.

Table 5

Year wise geographical distribution of CCHF cases in Pakistan

Year Reported cases Areas Mortality Reference
2000-02 191 Balochistan, Khyber Pakhtunkhwa, Punjab, Sindh 59 -58
2003-06 328 Kalat, Noshki, Balochistan, Khyber Pakhtunkhwa, Punjab, Sindh 42 -58
2009-10 29 Balochistan, Khyber Pakhtunkhwa, Punjab, Sindh 3 -58
2011-12 61 Balochistan, Sindh, Khyber Pakhtunkhwa, Punjab 17 -58
2013-14 134 Quetta, Qillah Abdullah, Zhob Balochistan, Sindh, Khyber Pakhtunkhwa, Punjab 27 -58
2015-16 131 Qilla Saifullah, Loralai, Chaghi, Balochistan, Sindh, Khyber Pakhtunkhwa, Punjab 44 -58
2017 23 Pishin, Ziarat, Mastung, Harnai, Musa, Khel, Balochistan - -57
16 Attock, Chakwal Faisalabad, Gujrat, Kasur, Mianwali, Multan, Muzaffarghar Rawalpindi (Punjab) - -57
7 Peshawar, Karak (KP) - -57
4 Karachi, Sindh - -57
1 Islamabad - -57
2018 63 Throughout the country 11 -53

Leishmaniasis

In Pakistan, leishmaniasis was first reported in 1960. It is widely spreading all over the country. No definite endemic area has yet been defined (59). However, the disease is reported along the entire western border extending north into North-West Frontier (N.W.F) down to southwest Balochistan and the northern area of Sindh. In Punjab, the epidemics have been reported in Multan, Lahore, and Dera Ghazi Khan (59, 60). The areas of Balochistan that showed the highest incidence of this disease are Gambaz, Maiwand, Kehan, Loherktak, Tali, Tangi, Spin, Dera Bugti, Sibi, Kohlu, Loralai, Fort Sandman, Khuzdar, and Lasbela. In Sindh, the disease had been reported from the Dadu, Larkana, and Jacobabad districts. Cutaneous leishmaniasis also occurred in northern areas (60), including Multan, Quetta, Lasbela, and Lahore. In 1960, Khaplu valley was the hotbed of the disease. In 1974, a new focal point was discovered in the Kharmang valley. In 1975, two cases were seen in the village of Parkuta. In 1979, the whole of Balochistan was surveyed and no cases of visceral leishmaniasis were found. There was a severe outbreak of cutaneous leishmaniasis in Quetta after the earthquake in 1935. Incidents of the disease in Balochistan are extensive. In 1974, the army personnel who were posted there contracted the disease (61). In 1971 another outbreak occurred in non-immune personnel in Uthal area of Lasbela. In Multan there was an epidemic in 1971-72. In Pakistan, the main reservoirs are dogs and rodents for urban and rural areas, respectively. To determine the species of sand fly in Pakistan, an entomological survey was undertaken in Balochistan. Three species were found: P. sergentii, P. Papatasii, and P. sergento myasquamipleur. P. sergentii was found to be the important vector in the causation of the disease (62). Two cases of visceral leishmaniasis were reported from Karachi (62, 63). Cutaneous leishmaniasis is common in Pakistan and outbreaks have occurred from 1996 to 2018 in different areas.

Figure 3  

Map showing the epidemiology of cutaneous leishmaniasis in Pakistan currently and diachronically.

Source: https://wdrfree.com/stock-vector/pakistan-map-with-districts with modification.

Table 6

Year wise geographical distribution of cutaneous leishmaniasis in Pakistan

Year Reported cases Area Reference
1996-2001 8007 Bela, Chaman, Dera Bugti, Dulki, Kohlu, Lasbela, Quetta and Sibi (64)
2002 5,000 Kurram Agency (65)
2007-08 355 Skardu (66)
349 Gilgit (66)
370 Chitral (66)
387 Dir upper (66)
395 Dir lower (66)
405 Swat (66)
446 Kohistan (66)
412 Batgram (66)
472 Mansehra (66)
484 Abbottabad (66)
800 AJK (66)
458 Rawalpindi (66)
343 D.G. Khan (66)
344 Rajanpur (66)
325 Jacobabad (66)
428 Larkana (66)
471 Dadu (66)
531 Lasbela (66)
485 Khuzdar (66)
517 Jhal Maghsi (66)
511 Dera Bughti (66)
514 Barkhan (66)
461 Musa Khel (66)
427 Zhob (66)
430 Qilla Saifullah (66)
415 Qilla Abdullah (66)
397 Pashin (66)
496 Quetta (66)
2009 1210 Larkana, Dadu and Jacobabad (67)
2013-2015 474 North and South Waziristan (68)
2017 94 Dadu, Hyderabad, Jamshoro, Sindh (69)
2018 2802 Bajaur (70)
5373 Mohamad (70)
9378 Khyber (70)
225 Kuram (70)
163 Orakzai (70)
443 North Waziristan (70)
354 South Waziristan (70)
270 Peshawar (70)
28 Lakki (70)
20 Kohat (70)
4 Tank (70)

West Nile fever

West Nile (WN) fever is a mosquito-borne disease. The West Nile virus had been isolated from humans and mosquitoes. During 1961-65, 5 strains of West Nile virus were isolated from culex tritaeniorhynchus and culex pseudovishni collected from Lahore and surrounding areas of Punjab (71, 72). During 1963-1964, two strains of West Nile virus were isolated from culex pipiens collected from Rawalpindi (73). In 1965, five human cases of West Nile virus were reported from Lahore and Rawalpindi (72). Sugamata et al. conducted a study in Karachi from 1983 to 1985 and isolated West Nile virus from a human blood sample (74). Another study showed isolated West Nile virus from Pakistani military personnel in 1996 (75). West Nile virus was also isolated from human samples collected from 2015 to 2016 (76). West Nile virus has been found in blood donors from Rawalpindi and Lahore in 2016 and 2017 respectively (76, 77, 78).

Japanese encephalitis

Japanese encephalitis is a mosquito-borne disease. The mosquitoes belonging to the genus Culex transmit Japanese encephalitis (79). In Pakistan, JE was confirmed in the early 1980s JE was isolated from a single specimen from Karachi in 1992 (80). No national Japanese encephalitis epidemiology is available due to the lack of a surveillance system.

Discussion

Due to the location and climatic condition Pakistan is a major hotspot of many vector-borne diseases. Malaria, dengue, CCHF and leishmaniasis are endemic and experiencing expansion of VBD’s in Pakistan. This literature review shows that malaria is endemic in all provinces except Punjab (31). The first dengue outbreak was recorded in Karachi but after 2000, the dengue outbreaks have been reported from Punjab, KP, Balochistan, ICT and AJK and other districts of Sindh (37-44,47). CCHF is highly endemic in the province Balochistan and cases were reported from Punjab (Attock, Chakwal Faisalabad, Gujrat, Kasur, Mianwali, Multan, Muzaffarghar Rawalpindi, KP (Peshawar, Karak), Sindh (Karachi) and Islamabad (53, 57, 48). Leishmaniasis is highly endemic in Balochistan and KP. Cases of leishmaniasis were also reported from northern region (Gilgit Baltistan and AJK), Sindh and Punjab province (64-70).

No studies have been done on the effect of climate change on the disease vectors (mosquitoes, ticks and rodents) in Pakistan. However, Dhmial et al (2021) conducted a study in 2020 and found that climate change is affecting the Hindu Kush Himalayan region (Nepal, Bhutan, Pakistan, Myanmar, Afghanistan, India, China and Bangladesh) due to changes in temperature, precipitation patterns and an increase in the frequency and intensity of extreme events. It was also found that the geographic range of VBDs expanded into previously non-endemic areas of mountains in the HKH region.

Maya N. et al (2015) conducted a study on impact of climate change on VBDs and found that the Mediterranean region (Egypt, Spain, Turkey, Italy, Malta and Israel) is vulnerable to climatic changes. As a result, it is expected that VBDs will be influenced by climate change in the Mediterranean region.

Conclusion

Climate change in Pakistan has affected the VBDs in both directions. Dengue, chikungunya, CCHF and leishmaniasis have been on the rise. There is no proper surveillance of vectors and diseases available in Pakistan. Early warning system for preventing outbreaks are needed. No study has been done on the effect of climate change on disease vectors in Pakistan. Long-term observational studies needed to monitor climate-change effects on vector borne diseases in Pakistan. A multidisciplinary effort is needed to handle the situation before the vector borne diseases becomes an epidemic in non endemic areas. More and more awareness should be created for the masses about the human activities that are causing global warming. To conclude, along with integrated vector management (IVM) for prevention and control of vector-borne diseases. It is essential to address issues related to climate change with the utmost priority.

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